Method and apparatus for the chemical analysis of materials by thermal decomposition



Delf. 3, 1940. QB FRANCIS ET AL 2,224,044

METHOD AND APPARATUS FOR THE CHEMICAL ANALYSIS 0F MATERIALS BY THERMALDECOMPOSITION Filed May 9, 1939 FIE.

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rPatentedl Dec. 3, 194() y I *,-UNITFED STATES PA'rlzmr oFFflcE MET'HOl)AND APPARATUS FOR THE CHEM- ICAL ANALYSIS OF MATERIALS BY THERMALDECOMPOSITION Charles B. Francis, Pittsburgh, Pa., and enry J W olthorn,Hubbard, and Truman S. Woodward, Youngstown, Ohio Application May 9,1939, Serial No. 272,713

1 claim. (c1. 23-230) This invention relates to a method and to apvalueof a solutionin thecontainer; c the novel 'paratus for the determinationof certain eleapparatus which constitutes the part of our inments inmaterials of various kinds by applicavention vpertaining to apparatus;and d-being a tion of heat to cause thermal decomposition of small'device of 'known and'suitable design for certain compounds under novelconditions in the the collection of dust and for the selectiveabpresence of a current of inert or slightly oxidizsorption by knownmeans of certain gases which ing gaseous mixture, and to an apparatusand itis desired to exclude'from the absorber appa` a method forreabsorbing the product from the kratus c. i Y l i stream of inert orslightly `oxidizing gas. Figure 2 illustrates arrangement of the essen-Ascan illustration of the use to which the ltial parts ofthe absorbingapparatus constitut- 10 method and apparatus of our invention can be inga part of our invention which may lbe sucapplied, we make use of theprocedure and apcessfully used-and may serve to illustrate the paratusfor the determination of sulphur in variprinciples of the operation andf construction of ous compounds, minerals and metals, such as, the partsnecessary to the successful carrying for example, sulphur in bariumsulphate, iron out of a determination.' A

ores, pig iron and steel. However, in the thermal ReferringparticularlytoFigure 2, tube l is for decomposition of materials, otherelements, such the purpose of admitting'gasesfrom the furnace as`fluorine, selenium and tellurium, may be tube into the absorber, whilepart 2 consists of evolved and determined in much the same mana bottomchamber capped `by a porousglass frit ner as in the determination ofsulphur, upon produced by a known process to permit the pas- 20 whichthe Vfollowing description of our method sage of bothiliquids and gases.The chief funcand apparatus is based. j tion of this part'in the presentapparatus is to The principles of our invention are illustrated break upthe gas stream from tube l into miby the accompanying'drawing, in which:nute streams or bubbles and thus prevent the gas Figure 1 is a sectionalelevation of an appafrom passing in large bubbles through any liquid 25ratus set-up suitable for carrying out the incontained in the chamberabove.` Platinum discs vention; and containing Very minute perforationsmay be sub- Figures 2 and 3 are sectional enlargements stituted fortheglass frit, as well as any similar taken from Figure 1, each' showingthe same substance havingthe same coefficienty of expanpiece ofapparatus 4at dierent phases of its sion as glass.` Likewise, tube viand part l2 may 30 operation. l be constructed ofany material inert tothe ac- In the draWing,A is a tank of nitrogen and tion of water;bases,and acids. A is a similar `tank of oxygen, or air under As indicated'bythe drawing', parts 3 and 4 pressure, the two tanks being connected at mperform the same functions in conducting liquids andthe gas from eachtank so regulated that and gases out' of the inner chamber` of the ab-35 any proportion of the two gases may be mixed sorber as parts 1i and 2do in admitting a gas as desired, oxygen being necessary in the analysisor liquid to the inner chamber. As will be more of materials, such aspigv iron and steel and some clearly 'explained' later, tube 5 isprovided to others.` However, it is a novel characteristic permit theforcing of liquids out of the inner 40 of our procedurethat the oxygens,in all events, chamber ,and jtube l for the escape of gases 40 dilutedor mixed with the nitrogen or with some only. 1 other chemically inertgas. Parts 9 and l0 form a novel type'of two-way The letter B representscommon equipment valve, ,the plug 9 being hollow and partly filledemployed for the purification of gases prior to with mercury to balancethe upward pressure their use in chemical reaction; C represents aagainst its bottom surface exposed to fluids con- 45 high temperaturecombustion furnace, specicaltainedwithin the inner chamber, andthe wallly the furnace described in Patent No. 1,903,036 of the plug 9 bearingindentations 8 andll so which issued to Charles B. Francis on March 28,thatvwhen it Vis turned in the position shown in 1933, includingthecombustion tube centrally the main drawing Figure 2, it will permitthe located in the furnace; D represents the assemescape of gas ythrough'l and 8- and the ingress 50 bly of apparatus required to separate anddeterof a `liquid from-the outer chamber to the inner mine the elementsfor which the material is chamber through the opening 6 in the shell andbeing analyzed, by the volumetric method; a tube 5, but when turned inthe position shown being an ordinary titrating burette; b apoteninFigure 3, .it lwill prevent the escape of gas tiometricinstrument for determining the pH from the inner chambervand permit aliquid to 55 Asulphur in steel and mineral substances.

flow out through tube 5 and opening 6, and when turned to anintermediate position will prevent the flow of both gases and liquidsexcept through the glass frit 2 and the tube 3 and frit 4.

To illustrate the method of procedure in making an analysis by themethod of our invention, we describe in detail the determination of Tomake a determination of sulphur in steel, a proper weight of sample istransferred to a combustion boat and inserted intoV the refractory tubeof the high temperature combustion furnace in the Well-known manner of acarbon determination by combustion, the various parts of the apparatusbeing-connected as illustrated in Figure 1.

Purified oxygen and nitrogen are admitted to the tube. All the Valvesvin the sulphur absorbing device of Figure 2 are closed and the chamberI5 is filled With water or a solution which hasfbeen adjusted to pH5,the same liquid also lling the Vouter chamber I4to a level slightlyabove thetop of the valve. Under these conditions, the gas mixturecontaining the gas to be absorbed ilows through the tube I and isseparated into an extremely large'number of minute streams by thefritted glass filter disc, these minute streams of gas rising as verysmall bub-v bles through the'liquid tothe top of absorber l5. Y"Underthe conditions'weemploy to burn the sample, namely, aV gas mixturewith an oxygen content belowfthat of air at a temperature of 1300ldegrees to 1350 degrees centigrade (approximately 2400degreesFahrenheit), the sulphur in the sample is evolved into the gasstream as SO; and is largely absorbed by the Water or Watersolutioncontrairiedain the absorber chamber I5. The proportion of oxygento the nitrogen or other inert gas Yshould `be sucient to eiectvdecomposition of lthesample but insuii'- cient to cause such a violentreaction as to result in spattering of the chemicals.

The mixture of gaswhich may containa` trace of'Sz or S03 not thusabsorbed, is carried through'tube 3 to the second glass frit and bubblesthrough the solution inthe outer chamber It.V By this secondopera'tionthe remainder of the gas is absorbed .and held by the liquid.

At this point, we turn the Valve in theV top of the absorber to theposition shown in Figure 3. I'he gas pressure then forces the liquidcontaining most of the S'Oznevolved in the determination through tubeS'and'openings II and 6 to the outer chamber I4. Without stopping theilow of gas, we then titrate the solution by the addition of astandardized alkali to the standard of 10H5, the'gas stream acting as ameans for stirring the solution during this titration.

AsV someof the acid SO2l gasfremains in the tube I, tube 3, frit 2 andfrit 4, as well as in the chamber l5, it is necessary to rinsetheseparts with some of thek solution in the chamber I4. To perform thisoperation, we shut off the gas stream from' the furnace and turn thevalve to the position shown in the main drawing or Figure 2. The gastrapped in chamber I5 then escapes through tubel tothe outlet 8, and theliquid from ,the chamber I4 flows through the openingB in the Valveshell and the by-pass II in the plug, to the tube 5. The hydrostaticpressure also forces some of the liquid through the fritted mats 4 andv2to iill the tube 3 and partly fill tube I. We then turn. the Valvedegrees the liquid out to I through frit 2 and out of tube 3 and throughfrit 4. When the gas is bubbling through frit 4, we turn the Valve 90Vdegrees to the vposition shown in Figure 3, allowing the liquid to ilowthrough tube 5 out into the chamber I4. Then we turn the valve 90degrees to close all openings and permit the gas to escape through frit4 and serve as a means for stirring or mixing the solution while it isagain titrated to an acidity of pH5. From the known amount of thestandard alkali solution used and its SOzequivalent, the per centsulphur in thesample may be found by simple calculation.

`With this apparatus and procedure, we are able to make anaccuratedetermination of sulphur, the -calculations being based upon theoreticalfactors calculated from molecular weights Without the use of empiricalfactors established by the use of standard samples.

For example, in the analysis of a steel for sulphur, we use an 0.01Nsolution of sodium hydroxide, the exact concentration of which has beenestablished by titration against an 0.01N solution of sulphuric acidthat has been carefully standardized by known chemical methods. Then tocheck our technique and the accuracy of our method and apparatus, weanalyze a known Weight of aA chemically pure compound, such'as sodiumsulphate, by the procedure described above.

Others have devised methods and apparatus for the determination ofsulphur in iron and steel by combustion in oxygen, but they have beenfound to possess inherent sources of error that have not heretoforeAbeen overcome.

Through ourfexperiments, we have discovered follows: A suitableanalytical sample is weighed` and transferred to the boat which is theninserted into the tube of the furnace. We then introduce a stream ofnitrogen if vthe mineral contains no combustible matter. If the mineralcontains combustible matter that should be burned to facilitate theanalysis, a limited portion'of' oxygen in the cheapest form, such aspurified air, is added with the nitrogen. This gaseous mixture ispassedthrough the tube of the furnace and through thespecial absorbingdevice which is manipulated'for the determination `of sulphur Vasdescribed above for the determination of sulphur in iron and steel. Theflow of the gas 'is continued until all lthe sulphur carried'by thesample has been evolved and absorbed in the SpecialV absorbingapparatus. The time required varies for dilerent substances andaccording to the amount of sulphur the substance contains and the formin which it is chemically combined. We have encountered no substancethat required a longer time than 30 minutes.

In .any case,ithe time required for analyzing an unknown substance isreadily ascertained to close all openings, andthe gas pressureforces byproceeding as follows The gaseous mixture g5 ispassed through theapparatus for a period of at least l5 minutes when the titration isconducted as described above. The gas ow is continued for another periodand the titration is repeated. When all the sulphur in the sample hasbeen evolved, passage of the gas through the water solution in theabsorber causes no change in the acidity as indicated by thepotentiometric apparatus.

`The other constituents we have attempted to determine by this methodhave all been acidic. If these elements form oxides during thedecomposition in the gas stream employed, which oxides cannot beseparated from SO2 before the ga-s stream reaches the absorber, any oneof several different procedures may be followed to determine the sulphurand the other constituents evolved. Thus, we may titrate the total acidswith a standard alkali solution, then transfer the solution to anothervessel and determine one or the other by known analytical methods. Fromthese results, the percentage of the undetermined constituent may becalculated. In another method, we may dissolve a compound in thesolution contained in the special absorber which compound or reagent iscapable of precipitating or otherwise removing one of the constituentsfrom the solution, thus making its effect neutral with respect to the pHvalue of the solution as determined by the potentiometer.

In a similar manner, it is possible to analyze a substance for alkalieswhich are volatilized at temperatures below 1400 degrees centigrade(2500 degrees Fahrenheit), the change made in the procedure being thatthe titration is made with a standard acid solution instead of astandard alkali solution.

By a modification of the apparatus to control the exit of gases from theouter chamber M shown in Figure l so that they may be passed withoutloss through another absorber, it is possible to determine both sulphurand carbon with the same sample and at the same time. The CO2 passes outof the solution and is absorbed in a succeeding apparatus which consistsof the drying train and a suitable absorber for CO2 for gravimetricdetermination. Similarly, by controlling the composition of the gasstream and the temperature of the reaction zone in the tube of thecombustion furnace, We are able by the novel method and apparatus of ourinvention to effect the separation of various elements from compoundsand mixtures by converting them into volatile compounds which may beabsorbedin water or in another suitable liquid or in a suitable watersolution contained in the special absorber of a type shown in Figures 2and 3 which may be preceded in the train by other absorbents to removeinterfering compounds. For example, silicon may be converted into thevolatile silicon fluoride, arsenic, antimony, iron chromium, phosphorusand tin into their volatile chlorides.

A particularly useful application of the method and apparatus ispossible in the analysis of gases. Thus, for the accurate determinationof the total sulphur in a gas mixture, a measured Volume of the gas ismixed with sufcient oxygen to effect complete combustion and the mixturepassed through the heated tube of the furnace with the apparatus of ourinvention connected as shown in Figure 1, and the sulphur dioxide in thegas and which is formed from the sulphur and sulphur compounds therein,is titrated with a standard solution of sodium hydroxide as describedfor the determination of sulphur in steel.

Another use is its application to the determinationv of inert elements,such as helium, nitrogen, argon, neon, and krypton, in gases. Thus', forthe accurate determination of helium and other inert elements ina fuelgas, we first ll the combustion tube (for this purpose, We use a tube ofsmall diameter) of the furnace shown in Figure 1 with oxygen and connectto it a closed form of our absorber lled with a strongly alkalinesolution of pyrogallol, and to the exit opening of the absorber, weconnect a gas burette of suitable size and form to collect by knownmethods any gases escaping from the absorber. We then pass a measuredportion of the fuel gas mixed with a proper excess of oxygen into thecombustion tube and permit the products of combustion to pass into theabsorber, collecting the residue of gases in the gas burette. Since theexcess oxygen and the products of combustion, consisting of Water,carbon dioxide, and other'acidic gases, are all absorbed by the solutionin the absorber, the inert gas may be readily identied by known methods,and its constituents separated if necessary, should it prove to be amixture of two or more inert gaseous elements.

By passing the gas through a drying tube and an absorber to abstractacidic gases before passing it into the combustion tube and inserting adehydrating tube and another to absorb CO2 and other acidic gasesbetween the combustion tube and our novel absorbing device to abstractthe excess oxygen, connected to a burette as described above, a completeultimate analysis of the gas can be made by the procedure described.

When the sulphur in a substance is in the form of sulphites orsulphates, the gas passed over the heated sample may be chemically inertbecause these compounds contain suiiicient oxygen to produce sulphuroxides which may be absorbed and determined by titratingr the absorbentliquid, the Weight of the sample being, of course, predetermined. Whenthe sulphur is elemental or in the sulphide form, for instance, it isnecessary to add oxygen to the inert gas, this also being necessary toeiect thermal decomposition of some substances, such as steel, forinstance. The oxygen should in such instances, be proportioned to theinert gas so as to effect conversion of the sulphur to its oxide formand decomposition of the substance, without causing a violent reactionor destruction of the furnace tube.

In the following claim it is to be understood that the sulphur in thematerial may be in either sulphate or sulphite form or in both forms,and

that the amount of sulphur present in either or both forms Will bedetermined by the method.

We claim:

A method of quantitively determining sulphur occurring in the form ofsulphates and sulphites in a material that is thermally decomposable,said method including heating a sample of said material in a chemicallyinert gas to a tempera` ture causing thermal decomposition of thesulphates and sulphites, and the production of sulphur dioxide gas,absorbing the sulphur dioxide gas, and measuring the amount of thesulphur dioxide absorbed. f l

. CHARLES B. FRANCIS.

HENRY J. WOLTHORN. TRUMAN S. WOODWARD.

